Balanced double ionization chamber X-ray monitor



BALANCED DOUBLE IONIZATION CHAMBER X-RAY MONITOR 2 Sheets-Sheet l L. J. RAINWATER n uu:

June 23, 1953l Filed May 5,y 195o INVENTOR- LEO JAMES RAINWATER VOL.I

June 23, 1953 1 J. RAINWATl-:R

BALANCED DOUBLE IONIZATION CHAMBER X-RAY MONITOR 2 Sheets-Sheet 2 Filed May 5, lSSO JNVENToR.- n Leo .nml-:s RAmwATr-:R

Patented June 23, 1953 BALANCED DOUBLE IONIZATION CHAMBER X-RAY MONITOR Leo James Rainwater, New York, N. Y., assignor to the United States of America. as represented by the United States Atomic Energy Commission Application May 5, 1950, Serial No. 160,324

Claims.

The present invention relates to an apparatus for monitoring X-ray peak voltage.

It is frequently necessary to duplicate the X- ray intensity produced by a certain source within very close tolerances. This becomes necessary, .for example, in analytical work with X-rays where the extent of absorption of X-rays is used to give an indication of the amount of a certain substance in a sealed containei` or the like. The intensity of X-rays emitted must be precisely duplicated from measurement to measurement if the results are to be compared.

There are two factors included in intensity measurements of X-rays. The first is the peak voltage, E, or the voltage applied between the electrodes to accelerate the X-ray producingr electrons. The second is the cathode current, i, or the number of electrons which are accelerated between the electrodes. The value of the intensity, I, is directly proportional to the tube current i and the control of z' does not cause a serious problem since it is easily adjusted and measured. If i is kept constant and E, the peak Voltage, is varied the variation of the intensity measurement, I, is much more rapid. For example, it has been found that with an E of approximately 124 kilovolts, I varies as E raised to the fifteenth power. Thus the main problem in maintaining constant source intensity reduces to one of maintaining E or the quality of the X-ray beam constant.

It is accordingly an object of the present invention to provide an apparatus suitable for measuring deviation from a certain intensity of X-rays in order to adjust their intensity to a certain value.

It is another object of the present invention to provide an apparatus capable of monitoring the intensity of X-rays produced from a tube with respect to a predetermined value.

It is still another object of the present invention to provide means which facili-tate readjustment of the X-ray intensity at a given value after a lapse of time.

Other objects and advantages of the present invention will be in part obvious and in part pointed out hereinafter.

In one of its broader aspects the objects of the present invention may be achieved by providing a container divided by an ion collector plate into a large and a smaller ioniza-tion chamber, disposing conducting surfaces in confronting relation to the faces of said ion collector plate, impressing la voltage on the confronting surfaces to make the potential on the ion collector plate (Cl. Z50-83.6)

intermediate to the potential on the confronting surfaces, and to cause ion currents to flow from each confronting surface to said collector plate, and providing means for detecting the difference in ion current flowing to said collector plate.

Figure 1 is a schematic view of the essential components of the apparatus of the present invention.

Figure 2 is a sectional View of a chamber arrangement suitable for effecting the purposes of the present invention taken on a line 2-2 of Figure 3.

Figure 3 is a horizontal section of the chamber of Figure 2 taken on la line 3--3 of Figure 2.

Figure 4 is a circuit diagram of the electrical system suitable for operating the present apparatus.

With reference particularly to Figure l a container |00 encloses three spaced parallel plates |02, |04, and |06. A source |08 of X-radiation is positioned with respect to the plates so as to emit X-radiation which passes through the plates in the directionnormal thereto before passing toward an object ||0. The plates |02, |04 and 06 are electrically connected respectively by the conductors ||2, ||4 and ||6 in electrically insulated relation through the container |00 to the voltage supply and detector unit |20 discussed below with reference to Figure 4. Electrical connection may be made between the source |08 and the unit |20 through the electrical conductors |22 .and |24 to regulate the source intensity in relation to a charge accumulated on the plate |04 as described below. This regulation is manual as described with reference to Figure 4.

The plate |02 is oppositely charged with respect to the plate |06 and the charge on plate |04 is intermediate to that on plates |02 and |05. For example, plate |02 may be positively charged with respect to plate |04 and the plate |06 negatively charged with respect to the plate |04. The radiation passing from the source |08 to the object |0 causes ionization of the gas within the chamber |00. The clearance between the plates |02 and |04 is smaller than the clearance between the plates |04 and |00. The relative clearance between the plates (indicated as volume 1 and volume 2) may be set at a desired value by adjustment of insulated screws 00 supporting the lower plate. If the intensity of radiation permeating the two volumes 1 and 2 were equal, a larger number of ions would be formed in volume 2 than in volume l because volume 2 contains more ionizable gas than volume 1. However, the intensity of radiation passing through volume 2 is relatively smaller than that passing through volume 1 because oi the absorption of radiation by the plate |04. By adjusting the clearance between the plates (by means of screws 98 for example) it is possible to balance and just offset the difference in ionization occurring in the two volumes as a result of the absorption of radiation by plate |04. Plate |04 collects ions of both posi-tive and negative charges from the two volumes because the plates |02 and |06 are oppositely charged with respect to the plate |04. Ii equal numbers of ions are formed in unit time in .the two volumes the number or" negative and positive ions collected on the plate |04 will be exactly equal and their charges will cancel. For one radiation intensity, therefore, no charge will collect on plate |04 and no current will iiow to or from the plate |04. By connecting plate |04 through the conductor I I4 to a very sensitive current indicating device, the voltage applied to the X-ray source |08 may be manually adjusted responsive to the presence or absence of a current owing from the plate |04. Reference is now made to Figures 2 and 3 wherein the details of a double ionization chamber, which has been found suitable for carrying out the present invention, are illustrated. The chamber` is formed between the cover |30 and a base |32. The cover and base are composed of an insulating plastic material. The chamber may be hermetically sealed by disposing a gasket |36 into the peripheral groove in the under surface of cover |30, and by pressing the peripheral tongue |34, extending upwardly from the base |32, into contact with gasket |36. Upper and lower metal plates |33 and |3I are held to the external surfaces of plastic members |30 and |32 respectively by screws |35. A set of bolts |38 extending through the members |30 and |32 and the external plates I3| and |33 are employed to urge the tongue |34 into the peripheral groove in the cover |30.

Within the chamber enclosed between the members 30 and |32, three disc electrodes are disposed in insulated relation to each other. The central disc |04 is maintained in insulated spaced relation from the upper disc |02 and the lower disc |06. The separation between the discs is not equal, the separation between the central disc |04 and the lower disc |06 being appreciably greater than that between the central disc |04 and the upper disc |02. The lower portion of the disc |04 may be hollowed out as shown to increase the separation between the discs. Disc |04 is maintained in its centrally spaced position by the sets of annular gaskets |40. The gaskets |40 and disc |04 may be held in position by the pegs |42 which are disposed in holes in the plate |04, gaskets |40 and members |30 .and |32. The upper plate |02 is held in position in the cover |30 by a snap ring |44 which ts into the conforming grooves in the confronting external and internal edges of the disc |02 and a depression in cover |30 respectively. A similar set of grooves and a snap ring |46 is provided for the base |06 to position it in the recessed portion of plate |32.

A chamber |50 containing detection apparatus, is provided adjacent the plate and disc assembly. This apparatus is described below with reference to Figure 4. The chamber |50 is surrounded by a protective shielding preferably of lead or similar heavy metal to minimize the amount of X-radiation penetrating the chamber |50 containing the instruments. Electrical connection is made between the electrical elements in the chamber |50 and plate |04 through the port |54. An auxiliary chamber |5| contains voltage supply means for supplying voltage to plates |02 and |06. Electrical connection is made between lower plate |06 and voltage supply means in chamber |5I through the port |56 in the base |32. A similar port in cover |30 permits electrical connection between plate |02 and the voltage supply in chamber I5 I The null point of the apparatus is the X-ray intensity for which the number of ions of opposite charge, being collected on the collector plate, just balances. Referring now to Figure 4, a detection circuit is shown which is suitable for use in conjunction with the balanced double ionization chamber described above to indicate the null point. The circuit consists of electrometer tubes 200 and 2|0, voltage supplies 202 and 203, voltmeter 2|2 and associated biasing and calibrating resistors. The iilaments 205 of the electrometer tubes are connected in parallel and supplied with lament voltage from battery 203 by means of conductors 206 and 201. This filament voltage may be adjusted by means of variable resistor 204. When the filament voltage adjustment is made, the meter 2|2 is connected into this circuit by placing the switch 2|4 in the down position. After this it may be switched to the up position except when checking the filament voltage. Plate voltage is supplied to the electrometer tubes from battery 202 through plate resistors 2|'I and 2|9 and conductors 2|6 and 2I8. Potentiometer 203 may be used for properly dividing this voltage between the two tubes. Resistor 220 is common to both tubes and is in series with the lament supply of voltage. Bias voltage on grids 2I3 and 2|5 is obtained from the voltage drop across resistor 220 and resistors 222 and 224 respectively. Voltage is supplied to plates |02 and |06 respectively from batteries 230 and 232 through conductors I2 and I I6.

The subject circuit is a standard two tube balanced circuit with common plate, filament .and grid bias voltages. A high degree of balance can be obtained when voltmeter 2 I 2 is connected across conductors 2 I 6 and 2 I8 by means of switch 2 I 4. Potentiometer 209 is varied until this meter reads zero. The plate voltage on the tubes has now been adjusted to exactly compensate for any differences in the tube characteristics or in the resistors that lare not common to both tubes. Once this adjustment has been made, changes in the voltage supplies will not affect the meter reading as the change in one tube will cancel the eiect of the change in the other.

The circuit can be unbalanced by a change in the voltage on grid 2|5 of tube 2|0. 'Ihis grid is connected to collector electrode |04 by means of conductor II4. As the charge on this plate varies due to change of X-ray intensity, current will flow through resistor 224 to ground. Resistor 224 is a resistance in the order of 101 ohms commonly used with electrometer tubes. The ilow of current through this resistor will change the voltage on grid 2|5 and cause a change in the plate current flowing through tube 2I0. The voltage across plate resistor 2I9 will change thereby causing voltmeter 2|2 to change its reading. If it is desired to maintain constant X-ray intensity, adjustment may then be made in the apparatus producing the X-rays, to cause the meter to resume its original reading.

As one illustration of an operable form of the apparatus described above with reference to Fig- 5 ures 2 and-3 a brass plate Ahaving a thickness of approximately 1g of an inch may be employed as the collector plate |04. An air gap of approximately 1% of an inch separates the plate |04 from the upper plate |02 and an air gap of approximately all, of an inch separates the collector plate |04 from the lower electrode |06. A potential difference of approximately 90 volts is established between plates |02 and |04 and an equal potential difference is established between the plates |04 and |06 so that thefvoltage between plates |02 and |05 is 180 volts. With this voltage and' in this configuration ofthe double ionization chamber it is found that a balanced condition or null point eXists for a value of E equal to about 135 kilovolts. The existence of a balanced condition or null point is indicated by an absence of a current ilowing to or from the collector plate |04 as indicated by a reading on meter 252. The sensitivity of the monitor to changes in E can be given almost any desired value by a choice of a suitably high input resistance. T-he sensitivity is proportional to the total X-ray intensity or the current i in the X-ray tube. A 20 microampere meter may be used to observe changes of 0.1 microampere in the null settings. With an X-ray emission current i of 1 milliampere and an input resistance of 1011 ohms, a change of 0.1 microampere in the output meter represents a change of 0.06 kilovolt or a 0.05 percent change in E. Greater sensitivity is readily obtainable by increasing the input resistance.

It is apparent from the foregoing description that a highly sensitive X-ray intensity monitor f scribed the housing of the ion current detecting k apparatus is conveniently arranged in a lead lined chamber adjacent the double ionization chamber, However, it is apparent that this particular arrangement or the use of the ion current detecting circuit illustratively disclosed is not essential to the carrying out of the present invention. Substantially any null point detecting apparatus such as one which counterbalances the current flowing from a collector plate, similar to plate |04, with the aid of `a null point electrometer, may be used in connection with the present invention. With regard to the double ionization chamber, substantially any conducting material may be used to form the plates |02, |04 and |06. The main criteria for the composition of plate |04 is that it be capable of absorbing sufficient X-rays to cause a change in the amount of ionization in the chambers between the plates. One way in which the null point of the apparatus may be varied is by variation of the material of which plate |04 is composed. Another method of varying the null point of the apparatus is by variation in the thickness of the collector plate used. Still a third manner in which the null point of the apparatus may be varied is by Varying the separation between the plates. It is also possible to make minor adjustments in the X-ray intensity, if the composition and thickness of the collector plate and the separations between the plates are kept constant,

6 by operating the chamber in such manner that a constant ion current flows to or from the collector plate as indicated by meter 2 l2.

The variation in the collector plate composition or thickness may be made by substitution of one plate for another. The variation in the relative clearance between the plates may be made by adjustment of screws such as those shown in the Schematic representation of the monitor in Figure 1. The Voltage on the plates 02 and |06 are preferably kept very close to equal and opposite with respect to the plate |04. However, minor variations in this voltage do not result in excessive deviations in the collector plate current.

Since many embodiments might be made of the present invention and since many changes might be made in the embodiment described, it is to be understood that the foregoing description is to be interpreted as illustrative only and not in a limiting sense.

I claim:

l. An X-ray intensity monitor which comprises in combination a container divided by an ion collector plate into a larger and a smaller ionization chamber, two conducting surfaces disposed in said container in confronting relation to the faces of said ion collector plate, a source of X- radiation spaced from the monitor in a direction substantially normal to the plates thereof thereby to direct said radiation through both the larger and smaller chambers consecutively, a source of voltage, means for impressing the voltage on said conducting surfaces and means for detecting a change in the charge on said ion collector plate.

2. An X-ray intensity monitor which comprises in combination a container divided by an ion collector plate into a larger and a smaller ionization chamber, two conducting surfaces disposed in said container in confronting relation to the faces of said ion collector plate, a source of X-radiation spaced from the monitor in a direction substantially normal to the plates thereof thereby to direct said radiation through `both the larger and smaller chambers consecutively, a source of voltage, means for impressing the voltage on said conducting surfaces so that the potential of the ion collector plate is intermediate between that impressed on the conducting surfaces and means for detecting a change in the charge on said ion collector plate.

3. An X-ray intensity monitor which comprises in combination a container divided by an ion collector plate into a larger and a smaller ionization chamber, two conducting surfaces disposed in said container in confronting relation to the faces of said ion collector plate, a source of X- radiation spaced from the monitor in a direction substantially normal to the plates thereof thereby to direct said radiation through both the larger and smaller chambers consecutively a source of voltage, means for impressing the voltage on said conducting surfaces to cause the potential of the ion `collector plate to be intermediate between that impressed on the conducting surfaces, means for detecting a change in the charge on said ion collector plate and means for adjusting the separation between the faces of said collector plate and said confronting surfaces.

4. A radiation intensity monitor which comprises in combination an hermetically sealable container enclosing an ion collector plate and two additional plates, one of said additional plates confronting said ion collector plate on either side but spaced from said ion collector plate by different distances to form two ion chambers of different depths, a source of X-radiation spaced from the monitor in a direction `substantially normal to the plates thereof thereby to direct said radiation through both the larger and smaller chambers consecutively a source of voltage, means for impressing equal and opposite voltage on the two outer plates and means for detecting the net ion current flowing to o1' from said ion collector plate.

5. A radiation intensity monitor which comprises in combination an hermetically scalable container enclosing an ion collector plate and two additional plates, said plates being circular and coaxial, one of said additional plates confronting said ion collector plate on either side but spaced from said ion collector plate by different distances to form two ion chambers of different depths, a source of X-radiation spaced from the monitor in a direction normal to the plates thereof thereby to direct said radiation through both the larger and smaller chambers consecutively a source of voltage, means for impressing equal and opposite voltage on the two outer plates and means for detecting the ion current flowing to or from said ion collector plate.

LEO JAMES RAINWATER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,383,820 Rosenblum Aug. 28, 1945 2,467,420 Binneweg, Jr. r Apr. 19, 1949 2,473,827 Spencer June 21, 1949 2,487,216 Blau et al. Nov. 8, 1949 2,531,804 Carlin et al. Nov. 28, 1950 FOREIGN PATENTS Number Country Date 319,202 Germany Apr. 14, 1918 

